The present invention relates to an apparatus and method for injecting fuel into a combustion chamber and in particular to a unitized fuel injector for engines that use overhead cams to actuate the injectors.
Diesel engines for locomotives use unitized fuel injectors that are actuated by overhead cams. One such typical conventional unitized injector is schematically represented in FIG. 1A and is generally designated by the numeral 10. This unitized injector 10 includes a valve body 11 that is disposed in an injector nut 29. The valve body 11 houses a needle valve that can be biased in the valve's closed position to prevent the injector from injecting fuel into one of the engine's combustion chambers, which is generally designated by the numeral 20.
As shown in FIG. 1B, which depicts an expanded cross-sectional view of a portion of the valve body 11 of FIG. 1A, the needle valve includes a conically shaped valve seat 12 that is defined in the hollowed interior of the valve body 11 and can be mated with and against a conically shaped tip 13 at one end of a needle 14. The hollowed interior of the valve body 11 further defines a fuel pathway 15 connecting to a fuel reservoir 16 and a discharge plenum 17, which is disposed downstream of the needle valve. Each of several exit channels 18 typically is connected to the discharge plenum 17 by an entrance orifice 19 and to the combustion chamber 20 by an exit orifice 21 at each opposite end of each exit channel 18. The needle valve controls whether fuel is permitted to flow from the storage reservoir 16 into the discharge plenum 17 and through the exit channels 18 into the combustion chamber 20.
As shown in FIG. 1B, the conically shaped tip 13 at one end of needle 14, which is housed in the hollowed interior of the valve body 11, is biased into sealing contact with valve seat 12 by a spring 22 (FIG. 1A). As shown in FIG. 1A, a cage 28 houses spring 22 so as to be disposed to apply its biasing force against the opposite end of the needle 14. A fuel pump 23 is disposed above the spring-biased end of the needle 14 and in axial alignment with the needle 14. Another spring 24 biases a cam follower 25 that is disposed above and in axial alignment with each of the fuel pump 23 and the spring-biased end of the needle 14. The cam follower 25 engages the plunger 26 that produces the pump's pumping action that forces pressurized fuel into the valve body 11 of the injector. An overhead cam 27 cyclically actuates the cam follower 25 to overcome the biasing force of spring 24 and press down on the plunger 26, which accordingly actuates the fuel pump 23. The fuel that is pumped into the valve body 11 via actuation of the pump 23 hydraulically lifts the conically shaped tip 13 of the needle 14 away from contact with the valve seat 12 and so opens the needle valve and forces a charge of fuel out of the exit orifices 21 of the injector 10 and into the combustion chamber 20 that is served by the injector.
However, the injector's exit orifices can become fouled and thereby adversely affect the amount of fuel that is able to enter the combustion chamber. Moreover, improving the fuel efficiency of these engines is desirable as is reducing unwanted emissions from the combustion process performed by such engines.
The goal of achieving more efficient combustion, which increases power and reduces pollution from the combustion process thereby improving the performance of injectors, has largely been sought to be accomplished by decreasing the size of the injector's exit orifices and/or increasing the pressure of the liquid fuel supplied to the exit orifice. Each of these solutions aims to increase the velocity of the fuel that exits the orifices of the injector.
However, these solutions introduce problems of their own such as: the need to use exotic metals; lubricity problems; the need to micro inch finish moving parts; the need to contour internal fuel passages; high cost; and direct injection. For example, the reliance on smaller orifices means that the orifices are more easily fouled. The reliance on higher pressures in the range of 1500 bar to 2000 bar means that exotic metals must be used that are strong enough to withstand these pressures without contorting in a manner that changes the characteristics of the injector if not destroying it altogether. Such exotic metals increase the cost of the injector. The higher pressures also create lubricity problems that cannot be solved by relying on additives in the fuel for lubrication of the injector's moving parts. Other means of lubricity such as applying a micro inch finish on the moving metal parts is required at great expense. Such higher pressures also create wear problems in the internal passages of the injector that must be counteracted by contouring the passages, which requires machining that is costly to perform. These wear problems also erode the exit orifices, and such erosion changes the character of the injector's plume over time and affects performance. Moreover, to achieve the higher pressures, the fuel pump must be localized with the injector for direct injection rather than disposed remotely from the injector.
Using ultrasonic energy to improve atomization of fuel injected into a combustion chamber is known, and advances in this field have been made as is evidenced by commonly owned U.S. Pat. Nos. 5,803,106; 5,868,153 and 6,053,424, which are hereby incorporated herein by this reference. These typically involve attaching an ultrasonic transducer on one end of an ultrasonic horn while the opposite end of the horn is immersed in the fuel in the vicinity of the injector's exit orifices and caused to vibrate at ultrasonic frequencies. However, unitized fuel injectors cannot be fitted with such ultrasonic transducers because of the disposition of the fuel pump, cam follower and overhead cam in axial alignment with the needle.